The disclosures herein relate generally to computer systems and more particularly to a shielding apparatus for reducing electromagnetic emissions from a computer system.
In order to meet electromagnetic emissions regulations required by agencies such as the Federal Communication Commission (FCC), it is common practice to place a compliant and conductive shielding device between an expansion device of a computer and the computer chassis. The shielding device provides a ground path between the chassis and the expansion device to aid in containing electromagnetic emissions within the chassis.
U.S. Pat. No. 5,653,396 discloses a grounding system for an electrical connector assembly which provides an interconnection between a PC card and a main printed circuit board. A header connector is mounted on the main printed circuit board and receives the PC card. Mating terminals are provided on the header connector and a receptacle connector on the PC card for interconnection of the PC card to electrical circuit traces on the main printed circuit board. At least one header grounding contact is mounted on the header connector adjacent to, but spaced apart from, the terminals and is coupled to a logic ground circuit on the main printed circuit board. A card grounding contact is mounted on the PC card near the front insertion end thereof for engaging the header grounding contact and for connecting the logic ground circuit of the main printed circuit board to a logic ground circuit on the PC card to provide a low impedance ground return.
U.S. Pat. No. 5,596,170 discloses a flexible electrical contact having a convex dome structure extending from one side of a surrounding structure. The convex dome structure and the surrounding structure form an integral metallic structure. The convex dome structure includes a central contact portion and a number of legs extending between the central contact portion and the surrounding structure. Each leg extends radially outward from the central contact portion and circumferentially around part of the central contact portion.
U.S. Pat. No. 5,563,450 discloses a peripheral card including a two-part metal cover mounting a printed wiring board on a U-shaped plastic frame. The wiring board is spaced from inside surfaces of the cover preferably by a pair of spring clips mountable on side edges of the wiring board. Each spring clip has an apertured if intermediate bight portion overlying, and in scraped contact with, ground contact pads on the board. Barbed-end spring fingers extend integrally from the bight portion preferably toward the board side edge to which it is affixed. The fingers are in scraped contact with respective ones of the metal cover parts as the metal cover parts are assembled together. The clips hold the board from movement in the cover and electrically ground any static electrical charges on the metal cover parts to the contact pads and to a ground socket in the frame. The clips electrically ground the metal cover parts to provide electromagnetic shielding of the electrical circuitry on the wiring board.
U.S. Pat. No. 4,780,570 discloses an electromagnetic interference (EMI) shield for situations which require heavy-duty wiping insertions, such as when a relatively heavy electronic assembly is to be removably inserted in a cabinet. Integral EMI strips of conductive spring material are riveted to opposite cabinet walls so as to be adjacent the wiping sides of the inserted electronic assembly. Each EMI strip contains integral longitudinally spaced projecting fingers formed to provide durable and highly reliable EMI shielding capable of withstanding many insertions and removals.
A common commercially-implemented shielding device for use with expansion devices in a computer includes a metal shield with a plurality of compliant spring members. Such expansion devices may include, for example, memory cards, sound cards, etc. The spring members extend from a surface of the shielding device and include free ends for engaged the expansion device. The shielding device is positioned between the expansion device and the chassis. The ground path is created by the compliant spring members engaging against the expansion device and the shielding device being engaged against, or being attached to, the chassis.
Shielding devices of this type are known to have a number of undesirable characteristics. First, these types of shielding devices are easily damaged as a result of one or more of the free ends getting caught in the expansion devices during either removal or insertion of the expansion devices into the chassis. This results in the spring members being distorted, broken or otherwise disabled. Second, to ensure sufficient grounding performance and reliability, it is common for the shielding device to include a plurality of spring members. This increases the likelihood of one or more of the spring members being damaged. The use of a plurality of spring members significantly increases the forces associated with deflecting the spring members. With sufficient force, the chassis can be distorted. The distortion may result in the shielding device interfering with cables being connected to the expansion cards in configurations where a cable connector is connected to the expansion device through an access opening extending through the shielding device. The distortion often results in a breach in EMI protection, allowing electromagnetic emissions to be radiated from the chassis.
A preferred material for shielding devices is a high performance material such as tin-plated beryllium copper. Beryllium copper offers exceptional resiliency as compared to low performance materials such as cold-rolled, low-carbon sheet metal. However, high performance materials are significantly more expensive than low performance materials. In cost sensitive applications, the relatively high cost of high performance materials precludes their use as the material for the shielding device.
Therefore, what is needed is a robust EMI shielding apparatus that reduces the potential for electromagnetic emissions from a chassis of an electronic device, that reduces the potential of damage to the shielding apparatus, chassis or expansion device during insertion and withdrawal of the expansion device and that is made of a low-cost material exhibiting less than preferred flexural characteristics.
Accordingly, in one embodiment, an electromagnetic shielding device uses physical configurations of structural elements to provide desired flexural characteristics. To this end, an apparatus for reducing electromagnetic emissions includes a shielding body and a pair of spaced apart contact members attached to the shielding body. A main spring member is attached to at least one of the contact members. The main spring member includes an elongated beam offset from the contact members and a plurality of spring fingers interconnecting the beam and at least one of the contact members.
A principal advantage of this embodiment is that the apparatus may be made from a low-cost material that exhibits less than preferred flexural properties.